f ' (x) represents the derivative of a function. Primarily used in calculus, the derivative is the rate of change on a nonlinear system.
If you recall from linear algebra, lines have a slope, as they have a constant rate of change (and the slope was defined as 'm' in y=mx+b).
Curved systems (like parabola and other complex functions) have varying rates of change, requiring the use of the derivative of a function.
